Serena Williams heads into her Wimbledon defence on the back of two major final losses, and Pete Sampras says a lot will depend on her inner drive to succeed.

Williams lost both the Australian and French Open finals earlier this year - to Angelique Kerber and Garbine Muguruza - with Wimbledon the only major crown remaining in her possession.

The world number one triumphed at the All England Club with a straight-sets win over Muguruza last year, and is widely tipped to retain her title despite her prior falterings in 2016.

And Sampras, who won Wimbledon in all but one year between 1993 and 2000, believes his fellow American's greatest threat is how much remains of her famous will to win.

"Well I just think her biggest obstacle is her motivation," he told Omnisport. 

"She's 34 and won over 20 majors. She's 34 now and has been around for many years, getting a little banged up and these girls are getting younger and better.

"She is still probably the best player in the world. It's her motivation and her drive. When you have been up there for so long it's tough to do that.

"She's done a lot for the game, she's a great player but as we get older it gets tougher and she's been around for 16-17 years and you're going to have your ups and downs but I look at Wimbledon and she's going to be right there.

"But she's an all-time great and she's done a lot for the game. It gets tougher as you get older, recovery time is tougher and you don't feel as fresh in the morning.

"It takes its toll, my last few years on the tour I felt tired and when I hit 30 I felt being out there on the road I just felt as though it was time to move on."